all: use local resolver for ADDC

For normal OS resolver, ctrld does not use local addresses as nameserver
to avoid possible looping. However, on AD environment with local DNS
running, AD queries must be sent to the local DNS server for proper
resolving.

cmd/cli/dns_proxy.go

@@ -51,6 +51,12 @@ var privateUpstreamConfig = &ctrld.UpstreamConfig{
 	Timeout: 2000,
 }
 
+var localUpstreamConfig = &ctrld.UpstreamConfig{
+	Name:    "Local resolver",
+	Type:    ctrld.ResolverTypeLocal,
+	Timeout: 2000,
+}
+
 // proxyRequest contains data for proxying a DNS query to upstream.
 type proxyRequest struct {
 	msg            *dns.Msg
@@ -443,6 +449,11 @@ func (p *prog) proxy(ctx context.Context, req *proxyRequest) *proxyResponse {
 		upstreams = []string{upstreamOS}
 	}
 
+	if p.isAdDomainQuery(req.msg) {
+		upstreamConfigs = []*ctrld.UpstreamConfig{localUpstreamConfig}
+		upstreams = []string{upstreamOS}
+	}
+
 	res := &proxyResponse{}
 
 	// LAN/PTR lookup flow:
@@ -651,6 +662,14 @@ func (p *prog) upstreamConfigsFromUpstreamNumbers(upstreams []string) []*ctrld.U
 	return upstreamConfigs
 }
 
+func (p *prog) isAdDomainQuery(msg *dns.Msg) bool {
+	if p.adDomain == "" {
+		return false
+	}
+	cDomainName := canonicalName(msg.Question[0].Name)
+	return dns.IsSubDomain(p.adDomain, cDomainName)
+}
+
 // canonicalName returns canonical name from FQDN with "." trimmed.
 func canonicalName(fqdn string) string {
 	q := strings.TrimSpace(fqdn)